The current virtuals system is decentralized; that is there is no way to find
information about a specific virtual other than to scan all packages for what
they provide. There is also no way to tell whether an atom is a virtual or
not - yes, the "virtual/" prefix could have been used but it isn't which has
led to its abuse.

What this means is that portage must scan all installed packages for the
virtuals they provide, that profiles must provide a default for every single
virtual that portage might encounter and that every single atom that portage
processes must be checked against the list of virtuals. Needless to say that
this causes quite a performance decrease.

The current virtuals system also has some other major shortcomings. The most
well known case is virtual/jdk and kaffe. Kaffe-1.1.4 implements the Java 1.4
API but can not satisfy a package that requires >=virtual/jdk-1.4 because
kaffe's versioning scheme differs. (ED: Need to add some more here. ;)

This GLEP recommends that virtuals become no different to regular packages.
Specifically, the standard virtual would include the DESCRIPTION, KEYWORDS,
IUSE and RDEPEND metadata. An example would be something like this:

Presently, it is very easy to remove packages even if others are dependent
on them, which can lead to broken emerges when packages rely on indirect
dependencies. For example, if kdelibs is merged bringing in qt and then
qt is unmerged, attempting to merge kdebase will likely fail. This becomes
a much bigger problem with virtuals as packages because the dependencies
are always indirect.

The resolution for this issue will be to add full dependency tracking and
verification to portage. The details of how it will be done are outside the
scope of this GLEP, but essentially this means that portage will need to be
forced to unmerge a package that is depended on by another and will also be
able to scan and fix any broken dependencies.

Profiles currently specify the default provider of each virtual and users are
able to override these defaults using /etc/portage/profile/virtuals. If
virtuals are replaced by regular packages and thus able to have arbitrarily
complex DEPENDs, the current method of overriding default virtuals can not
be extended to support this.

Before looking at a solution, lets look at how the current system works. When
portage initializes, it searches installed packages for available virtuals.
It then searches profiles and user overrides and adds them to the available
providers list and/or changes the order of the providers so that overrides are
listed earlier. Portage then expands any virtual atom it finds into an OR
list using the order decided upon at initialization.

To keep this behaviour available, this GLEP proposes a new file named
package.prefer. In its basic form, this is just a list of package names
ordered by preference. Portage would use it by reordering the atoms of any
OR list it processes to fit the order given by package.prefer. For example,
if package.prefer contained "dev-java/kaffe" then:

|| (
dev-java/blackdown-jdk
dev-java/sun-jdk
dev-java/kaffe
)

would be processed as:

|| (
dev-java/kaffe
dev-java/blackdown-jdk
dev-java/sun-jdk
)

In its basic form, package.prefer already covers profile and user overrides.
However, this GLEP proposes that any type of atom be usable. This will be
accomplished by checking for intersections of the atoms in the OR list and
atoms in the preferred list. When an intersection is found, both atoms
would be specified in a sublist, which would then be treated as a ranged dep.
For example, if package.prefer contained "<=dev-java/sun-jdk-1.4" then:

The number one advantage is that it offers more power to both the user and
the developer. Flexibility of virtuals is far greater in this scheme and
fulfills requirements that exist already. It also means that the maintainers
of profiles will not need to list a default for every virtual. The user
benefits by being able to easily gather a list of providers of a virtual as
well as their control being extended to allow selection where there is a
choice within any package.

Portage code also benefits from this scheme as virtuals will no longer
require special handling or dual implementations of essentially the same
feature, for example USE-based PROVIDEs. This scheme is also much easier to
optimize which will benefit the processing of all packages. It also means
that any additions to the DEPEND vocabulary become available for use in the
definitions of virtuals.

Compatibility will begin by making 2.0.51.20 treat unknown virtuals like
regular packages. When the tree is stripped of PROVIDEs and "virtuals"
override files, the only virtuals that these portages will use are those that
the user has specified and those gleaned from installed packages. Any
unknown virtual will be treated like a regular package and looked for in the
tree.

The next major version of portage (2.1.0) will support consistency
checking. The only remaining issue is that of user overrides. The old
method will work even with new style virtuals. The only catch is that
complex virtuals - that is virtuals that will install more than one package -
may not be overridable satisfactorally.

Dropping of support of current style virtuals is planned for the following
major version of portage (2.2.0). When the time comes to release it, scripts
will be written to create packages from the existing virtuals system as well
as to create appropriate package.prefer overrides within the profiles.